Most normal galaxies have little or no radio halo, with the
radio surface brightness
perpendicular to the disk being best described by exponential
functions with a median scale height of ~ 1 kpc
(Beck, 1997b).

Bright, extended radio halos around spiral galaxies are very
rare - exceptional,
rather than normal. One of these rare galaxies with an extended
halo is NGC 4631, with a
scale height of ~ 2 kpc, magnetic field lines going vertically out
of the galactic disk, possibly due to a galactic fountain or galactic wind
(Hummel et al., 1991).

Future trends. Little is known for sure on magnetic
field strengths and shapes in
halos of spiral galaxies. Very preliminary data have been reviewed in
Beck et al. (1996) and
Beck (1997b),
and some early comparisons were made with galactic disks
(Beck, 1997a).

In the case of M51,
Berkhuijsen et al. (1997)
used a multi-layer 3-dimensional
model for the position angles of the polarization at 4 different
radio wavelengths, smoothed
to a beam of 3.5 kpc. Their method involves the Fourier
parameterization of the magnetic
field structure into many radial, azimuthal, and vertical
components inside each layer. They
found a small predominence of the bisymmetric (BSS) global
magnetic field shape in the
galactic disk, notably for a radial distance < 6 kpc. They also
found a predominence of the
axisymmetric magnetic field shape in the galactic halo facing us,
possibly due to
a topological pumping of magnetic field by a galactic fountain
flow. They could not say
anything about the magnetic field in the side of the halo on the
other side of the disk, since the nearly face-on galaxy is not
transparent at 20cm. The
general features of the magnetic
fields seem to be in general agreement with predictions from
dynamo theory, but detailed
dynamo modelling is required to reach definite conclusions.